Liquid crystals are used in a wide variety of devices, including visual display devices. The property of liquid crystals that enable them to be used, for example, in visual displays, is the ability of liquid crystals to transmit light on the one hand and to scatter light and/or absorb it (especially when combined with an appropriate dye) on the other, depending on whether the liquid crystals are in a de-energized or field-off state, or in an energized or field-on state. An electric field selectively applied across the liquid crystals may be used to switch between the field-off and field-on states.
There are three categories of liquid crystals, namely cholesteric, nematic and smectic. The present invention relates in the preferred embodiment described hereinafter to the use of liquid crystal material which is operationally nematic. By "operationally nematic" is meant that, in the absence of external fields, structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal at its boundaries rather than by bulk effects, such as very strong twists (as in cholesteric material) or layering (as in smectic material). Thus, for example, a liquid crystal material including chiral ingredients which induce a tendency to twist but which cannot overcome the effects of the boundary alignment of the liquid crystal material would be considered to be operationally nematic.
A more detailed explanation of operationally nematic liquid crystal material is provided in U.S. Pat. No. 4,616,903, issued Oct. 14, 1986, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to U.S. Pat. No. 4,435,047, issued Mar. 6, 1984, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, and which disclosure is also hereby incorporated by reference.
Nematic curvilinear aligned phase (NCAP) liquid crystal and devices using NCAP liquid crystal are also described in the above-identified U.S. Pat. No. 4,435,047. An NCAP film may comprise a containment medium containing plural volumes of operationally nematic liquid crystal. The plural volumes may be discrete or interconnected capsules. As described in U.S. Pat. No. 4,707,080, issued Nov. 17, 1987, entitled ENCAPSULATED LIQUID CRYSTAL MATERIAL, APPARATUS AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference, the interconnecting channels or passageways may also contain liquid crystal material.
A functional NCAP liquid crystal device may consist of NCAP liquid crystal sandwiched between two electrode-coated substrates. A voltage source may be connected between the electrodes to selectively apply an electric field across the liquid crystal material. A pleochroic dye may be present with the liquid crystal material to provide substantial attenuation by absorption in the field-off state but to be substantially transparent in the field-on state.
An optical reflector may be located to the rear of the NCAP film. The reflector may be colored. Reflection by the device will b a function of whether there is an electric field applied across the liquid crystal material.
The optical performance of such a guest host display may be measured by comparing its luminance and color in the field-on and field-off states. The display's luminance (Y) is a quantitative measure of the intensity of light leaving the display surface. Its brightness is closely related to luminance but is not quantitative. The display's color may be represented by the Cartesian coordinates (x,y) on a 1931 C.I.E. chromaticity diagram.
Ideally, in the field-off state, the display's luminance should be low and its color as close to a colorless, neutral state as possible. In the field-on state, just the opposite is preferably true. The display's luminance should be high, and its color, which is dependent upon the reflector's color, should be very saturated.
As is known, the display may be made darker (lower luminance) in the field-off state by using a high concentration of dye or a thicker liquid crystal film. However, as a result for a given voltage, in the field-on state, the display is also darker and its color less saturated. Thus, although the optical performance of the display may be improved in the field-off state, it is no better, indeed it is worse, in the field-on state.
A potential solution to this problem has been the use of higher performance pleochroic dyes, i.e. dyes with a higher order parameter. However, even with such dyes there is a direct trade-off between the optical performance in the field-on and field-off states. That is, the optical performance in the field-off state cannot be improved without adversely affecting that performance in the field-on state and vice-versa. Thus, there is a limitation on the degree of color contrast and luminance that may be obtained in the field-on and field-off states.
Accordingly, an object of the present invention is to enhance the optical performance of a liquid crystal display wherein the display's field-off and field-on performances are more independent of each other.
A more specific object of the present invention is to provide a guest host display that has enhanced backscattering in the field-off state but enhanced light transmission in the field-on state.
Yet another object of the present invention is to provide a guest host display that has improved color contrast and luminance.